Hydrostatic auxiliary wheel motor assist drive system for vehicles

ABSTRACT

A rough terrain vehicle has a conventional engine-driven primary drive system and an auxiliary hydrostatic assist drive system having a higher ratio drive than the primary drive system. Thus in the assist system, wheel motors in the normally undriven wheels provide traction assist for the primary drivers only when and as needed. An engine-driven pump supplies pressure fluid to the wheel motors through the pivot joint between the vehicle&#39;&#39;s steering axle and the wheel spindle trunnions. A manually operated forward-reverse master control valve selectively energizes a hydraulic control circuit for the assist system including pressure-operated power control and shorting valves to ensure adequate supply of fluid to the wheel motors to meet demand under various modes of operation. The circuit enables five modes of operation, including a neutral mode, powered forward and reverse modes, a retard mode and an overrun mode. A motor circuit pressure-operated clutch modulator valve controls the engagement of a fluid activated friction clutch between each wheel motor and its driven wheel in a manner so that the clutch is variably engaged or disengaged automatically as dictated by the need for traction assist.

United States Patent [191 Schwab et a1.

[ HYDROSTATIC AUXILIARY WHEEL MOTOR ASSIST DRIVE SYSTEM FOR VEHICLES [75] Inventors: Delmar G. Schwab; Dennis P.

Graham, both of Milwaukie, Oreg.

[73] Assignee: Hyster Company, Portland, Oreg.

[22] Filed: Apr. 23, 1971 21 Appl. No.: 136,741

[52] US. Cl. 180/44 M [51] Int. Cl B60k 17/34 [58] Field of Search 180/44 M, 44 F, 66 R [56] References Cited UNITED STATES PATENTS 3,584,698 6/1971 Larson 180/44 M 3,129,781 4/1964 Stein l80/44 F 3,057,427 10/1962 Glasgow..... 180/44 M 3,354,977 11/1967 Swift 180/44 M 3,480,099 11/1969 Nighswonger..... 180/44 M 3,481,419 12/1969 Kress 180/44 M [451 Dec. 25, 1973 [57] ABSTRACT A rough terrain vehicle has a conventional enginedriven primary drive system and an auxiliary hydrostatic assist drive system having a higher ratio drive than the primary drive system. Thus in the assist system, wheel motors in the normally undriven wheels provide traction assist for the primary drivers only when and as needed. An engine-driven pump supplies pressure fluid to the wheel motors through the pivot joint between the vehicles steering axle and the wheel spindle trunnions. A manually operated forwardreverse master control valve selectively energizes a by draulic control circuit for the assist system including pressure-operated power control and shorting valves to ensure adequate supply of fluid to the wheel motors to meet demand under various modes of operation. The circuit enables five modes of operation, including a neutral mode, powered forward and reverse modes, a retard mode and an overrun mode. A motor circuit pressure-operated clutch modulator valve controls the engagement of a fluid activated friction clutch between each wheel motor and its driven wheel in a manner so that the clutch is variably engaged or disengaged automatically as dictated by the need for traction assist.

27 Claims, 5 Drawing Figures 3:- I DELMAR e. SCHWAB JQLZ DENNIS F? GRAHAM BUCKHOfi/V, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS PATENTED B6225 I973 BUCKHO/W, 510/1 5, KLAROU/ST & SPAR/(MAN ATTORNEYS PATENTED DECZ 51975 I so 7 BUCKHOR/V, BLUFF, KLAROU/ST 8 SPAR/(MAN ATTORNEYS HYDROSTATIC AUXILIARY WHEEL MOTOR ASSIST DRIVE SYSTEM FOR VEHICLES BACKGROUND OF THE INVENTION cles primary drivers in moving the vehicle when needed.

DESCRIPTION OF THE PRIOR ART Auxiliary traction assist wheel motor drive systems for rough terrain vehicles are well known. An example of one such system is shown in Budzich U. S. Pat. No. 3,272,279. However, a deficiency of prior such systems is that the output torque of the wheel motors does not vary in response to the demand for additional tractive effort. For example, in the Budzich auxiliary drive system, the hydraulic pressure applied at the wheel motors remains relatively constant despite a varying demand as dictated by variations in vehicle speed relative to engine speed.

Other common deficiencies of prior auxiliary drive systems include: (1) the need of stopping the vehicle before engaging and disengaging the auxiliary drive; (2) inability to use the auxiliary drive in a directional mode opposite that of the primary drive for retarding vehicle motion; and (3) lack of motor protection against cavitation when the motors are back-driven by the wheels as pumps.

One of the primary applications of this invention is in industrial trucks operated on rough terrain. In this application auxiliary hydraulic wheel motors are normally operated under conditions which would produce cavitation in the absence of some sort of cavitation protection because of the great frequency of sharp turns and the frequent transition between soft ground and hard roadways and between light and heavy loads.

In prior auxiliary drive systems, the problem of protecting the hydraulic wheel motors against damaging cavitation is commonly met by providing an overpowered drive system as opposed to an underpowered system. That is, the auxiliary drive system is provided with less reduction or a lower ratio drive than the primary drive system together with a high capacity pump and excess power to ensure adequate oil supply to the wheel motors under all conditions including high angle turns. As a result, when such auxiliary drive systems are engaged, the auxiliary drivers become the primary drive wheels, and the main drivers retard the driving action of such auxiliary drivers. The overpowered auxiliary drive system also uses power inefficiently and provides a low tractive effort in relation to the power available. Stated differently, such prior auxiliary drive systems are not true assist systems in that they take over the function of the primary drivers rather than assist the primary drivers in their effort to move the vehicle.

A control intertie between the primary and auxiliary drive systems such as manual or mechanical interlocks or cutouts are commonly used in various operational speed ranges of prior drive systems to disengage one of the two systems when the other is operating to obviate the foregoing problems. However, in drive systems with such interties, the auxiliary drive is either fully engaged or fully disengaged and not acting as a true assist or supplement to the primary drivers. Such interties also complicate the drive systems considerably and can be subject to malfunction.

It is also common practice to connect the wheel motors to the hydraulic fluid supply in such a way that supply and return hoses are exposed to damage by rocks, brush and other road and off-road hazards. Such prior hose connections also limit wheel movement and thus the turning radius of the vehicle.

SUMMARY OF THE INVENTION In accordance with the present invention, an auxiliary hydrostatic traction assist wheel motor drive system automatically providesa variable tractive effort in response to varying vehicle demand when such auxiliary system is engaged.

A primary feature of the invention is the provision of an underpowered assist drive system having a higher drive ratio, or greater reduction, than the primary drive system. The advantages of such an assist system are (I) it provides a true assist drive which helps the primary drivers move the vehicle only when there is slippage in the power train of the main drivers rather than a primary or four-wheel drive type assist drive system; (2) it provides maximum tractive effort for the available power; (3) it provides for highly eff cient transfer of power to and utilization of power by the assist wheels; (4) it eliminates power loss in the absence of full power demand by the assist drive system; (5) it reduces loading on the drive system; and (6) it eliminates the need for any precise adjustment of gear ratios between the primary and assist drives.

Another primary feature is the provision of a hydraulic control circuit for the assist drive system which ensures that the hydraulic assist wheel motors will not demand more hydraulic fluid than the available supply with the assist system energized despite the greater reduction of the assist drive than the primary drive, even in the absence of slippage in the primary drive train. This feature has the advantage of permitting sharp angle turns and high speed operation with the assist system energized without damaging the assist wheel motors.

According to another feature of the invention, the auxiliary drive system can be engaged and disengaged while the vehicle is traveling at any speed.

In a more specific aspect of the invention, with the auxiliary drive system energized, a fluid-activated clutch between each wheel motor and its assist wheel is automatically engaged or partially disengaged in response to variations in differential pressure across the wheel motors. As each auxiliary drive wheel approaches the flow limit of its wheel motor, the reduced pressure drop of the system at least partially disengages the clutch to prevent such wheel from driving the motor as a pump.

A further feature of the invention permits engagement of the auxiliary drive system in a directional mode opposite that of the primary drive system to retard vehicle motion, such as when descending a steep grade.

Another feature provides a remote variable retard control to enable the operator to vary the degree of retarding action when the auxiliary drive system is in its retard mode.

When the wheel motors tend to act as pumps, such as when operating in-their retard mode, during sharp turns or in the event of overrunning, another feature of the invention in the hydraulic control circuit for the auxiliary drive system protects the wheel motors against cavitation by recirculating fluid flow in the motor circuit from the output to the input side of the motors and at a higher than usual pressure.

A pilot circuit in the auxiliary hydraulic control system is a feature which increases the pressure level in such system in the retard or overrun mode to ensure adequate supply of fluid at the motor inlets.

An object and advantage of the control circuit for the assist drive of the invention is the elimination of all control interties between the primary and assist drive systems formerly used to protect the drive systems in various speed ranges. V

In another aspect of the invention, hydraulic fluid is routed to and from the wheel motors and their clutches through a rotary seal in the kingpin or spindle trunnion connection to the steering axle. This feature minimizes exposure of hydraulic lines to the hazards of the terrain in which the vehicle operates without restricting the turning radius of the vehicle.

DESCRIPTION OF THE DRAWINGS The foregoing and other features, objects and advantages of the present invention will be more apparent from the following detailed description which proceeds with referenceto the accompanying drawings wherein: FIG. 1 is a schematic plan view of a rough terrain lift truck incorporating the present invention;

FIG. 2 is a vertical sectional view on an enlarged scale taken approximately along the line 22 of FIG.

FIG. 3 is a diagram of the hydraulic control circuit of the invention showing the circuit in its neutral mode;

FIG. 4 is a diagram similar to that of FIG. 3, but showing the circuit in its powered forward mode; and

FIG. 5 is a diagram similar to those of FIGS. 3 and 4 showing the hydraulic circuitin its retard mode.

DETAILED DESCRIPTION General Arrangement With reference to the drawings, FIG. 1 discloses a typical fork lift truck adapted for rough terrain use. The truck has front drive wheels 12 powered by an internal combustion engine 14 through a torque converter 16, transmission 18, differential and drive axle22. The truck also includes normally undriven rear steering wheels 23 pivotally connected to a steering axle 24 at a kingpin or spindle trunnion connection 26. Although the rear wheels are not normally driven, each carries a wheel motor 28 that serves as a boost drive or traction assist device when the vehicle needs more traction than the main drive wheels 12 are capable of providing in negotiating rough or steep terrain.

Engine 14 also drives a hydraulic pump means 30 which supplies hydraulic wheel motors 28 with pressure fluid. The pump may also supply other hydraulic components of the vehicle.

Referring to FIG. 2, each wheel motor 28 is of the hydrostatic type and includes a cartridge 32 housed within a pocket in the rear portion of wheel spindle 34. Pressure fluid flows to a rotary portion 36 of motor cartridge 32 through a forward passage 38 in wheel spindle trunnion 40.-A similar reverse passage 42 in trunnion 40 leads pressure fluid from the cartridge. Pressure fluid from pump 30 flowsthrough a hydraulic hose 44 on top of the steering axle 24 and through a unique rotary joint 46 at kingpin connection 26 into forward passage 38. A similar rotary joint 38 routes hydraulic fluid from the wheel motor reverse passage 42 back to pump means 30 through a hydraulic hose 50.

Rotary Joints Describing the rotary joints in more detail, with reference to upper joint 46, the upper end of the spindle trunnion 40 extends upwardly within an opening 52 within an upper flange portion 53 of steering axle 24. A cap member 54 into which the outer end of hydraulic hose 44 is threaded fits over and extends partly within opening 52 of axle flange 53. Cap 54 has an internal passage 56 communicating with hose 44 and trunnion passage 38. Cap passage 56 interconnects trunnion passage 38 at a freely rotatable sleeve member 58 having an internal connecting passage 59. The upper portion of sleeve 58 extends within passage 56 of cap 54, whereas the lower portion of such sleeve extends downwardly into passage 38 of trunnion 40. Suitable bushings, one 60 secured to the upper end of trunnion 40 and the other 61 press-fit within axle flange opening 52, provide for relative rotation between the upper end of the spindle trunnion and the axle. Suitable seals as shown are provided between sleeve member 58 and the cap and trunnion, between the cap and axle member and between the trunnion and axle to prevent leakage of fluid at these points.

The lower rotary joint 48 is similar in principle to upper joint 46 just described. However, in a slight variation of the mounting arrangement, tapered roller bearings 63 provide for relative rotation between the lower end of the spindle trunnion and a lower cap member 64. Nevertheless a freely rotatable lower sleeve member 66 similar to upper sleeve 58 has an internal passage connecting trunnion reverse passage 42 with a passage 68 in lower cap 64.

With pressure fl uid being fed to and from wheel motor 28 through the rotary connections between the wheel spindle trunnion and the steering axle, the hydraulic hoses and passages do not interfere with thesteering function of the wheels and axle. Moreover, such an arrangement minimizes the use of exposed hydraulic hoses. As shown, hoses 44 and 50 are well shielded by the axle itself from terrain hazards.

Clutch Assembly A rotary clutch means, indicated generally at 70, transmits torque from hydraulic wheel motor 28 to a planetary gear train of conventional design indicated generally at 72 and mounted within wheel hub 74. R0- tary clutch means includes a fluid-activated and cooled disc-type friction clutch pack 76 connected to a clutch shaft 78 extending through wheel motor cartridge 32 and terminating at its rear end at a clutch piston 80. Clutch shaft 78 is splined to rotary element 36 of wheel motor 28 but is adapted to slide axially through the wheel motor cartridge to close up and release the clutch pack, depending on the fluid pressure at clutch piston 80. Clutch piston 80 is housed within a spindle cover 82 closing the spindle pocket which houses the wheel motor cartridge.

The cover also houses a clutch modulator valve 84. This valve responds to pressure drop across the wheel motor in a manner so as to control the fluid pressure at the clutch piston and thereby determine when and to what extent the normally disengaged clutch pack will be engaged.

The clutch pack, when engaged, transmits torque from clutch shaft 78 to planetary shaft 86. Torque transmitted by the latter shaft through planetary gear train 72 drives wheel 23 in a direction determined by the direction of flow of fluid through the wheel motor. The operation of the modulator valve will be described in greater detail hereinafter.

Fluid admitted to clutch piston 80 through valve 84 flows through an orifice 85 in the piston and through internal passage 88 in clutch shaft 78 to clutch pack 76 and into the planetary housing to lubricate and cool these elements.

Wheel Motor Control Circuit With reference to FIG. 3, the hydraulic wheel motor control circuit includes a master control valve means comprising a three-position spool valve 100 springcentered to its neutral position 101 and movable manually to its forward position 102 and reverse position 103. Pressure fluid is routed to master valve 100 from fixed displacement pump 30 through line 104. In the neutral mode, pressure fluid returns to sump 105 through return line 106. A master relief valve 107 determines the maximum pressure in loop 104 and also in the motor circuit in the powered forward and reverse modes of operation.

The circuit also includes a charge valve assembly 1 which is preferably incorporated in a junction block installed, for example, on steering axle 24. The charge valve assembly provides cross-porting to divide hydrau- 'lic flow to each wheel motor.

The charge valve assembly has a forward supply port 111 through which pressure fluid is directed from master control valve 100 through passage 112 to a forward power control valve spool 114. Forward charge spool 114 is connected in parallel with a forward spool bypass line 116 having a forward supply check valve 117. A forward supply balance orifice 118 is connected in parallel with the forward supply check valve. Forward power control spool 114 incorporates a shorting valve means or portion 120 including a shorting passage 120a and a shorting orifice 121.

The charge valve assembly also has a reverse supply port 123 through which pressure fluid from master control valve 100 is directed in the reverse mode into a charge passage 124 to a reverse power control spool 126. The reverse charge circuit is identical to the forward charge circuit just described, and includes reverse supply check valve 128 in a reverse spool bypass line 135 and a reverse supply balance orifice 129, both in parallel with reverse power control spool 126.

Like the forward power spool 114, reverse power spool 126 incorporates a shorting valve portion 130 including a shorting orifice 131.

Alternatively, forward and reverse check valves 117, 128 and forward and reverse balance orifices 118, 129 could be replaced by an equivalent shuttle valve and orifices all within a common shuttle valve spool. Similarly, the shorting orifices 121, 131 could be provided externally of their respective spools 114, 126.

The charge valve assembly also includes a pilot circuit designated generally at l34and including a pilot relief valve 136 and pilot bleed orifice 138. The pilot 6 circuit 134 functions when the forward or reverse power spool is in its shorting range, that is, when there is flow through either shorting valve passage a or a, to build up sufficient pressure in the motor circuit to ensure adequate filling of the motors before permitting fluid to be metered through one of the power spools back to the reservoir.

The charge valve block also includes forward motor supply ports 140, 140a and reverse motor supply ports 142, 1420. The forward motor supply ports connect with lines 144, 144a leading to the forward inlet ports of the rightand left-hand wheel motors 28 and 28a respectively. The reverse motor supply ports connect with lines 146, 146a leading to the reverse inlet ports of the rightand left-hand wheel motor respectively.

A motor case drain port 148 in the charge valve block connects with motor case drain lines 150, 151,

152 to direct drain fluid through passage 154, motor case drain tank port 156 and line 158 back to the reservoir. Motor case drain lines 150, 151 also drain the clutch packs 76, 76a and the planetary gear trains 72, 72a.

Clutch modulator valve 84, 84a for the two wheel motors are responsive to differential pressure across the wheel motors 28, 28a as sensed through pilot lines 160, 160a from the forward side of the wheel motors and through pilot lines 162, 162a from the reverse side of the wheel motors. In the FIG. 3 control circuit the rightand left-hand clutch pistons 80 and 80a receive pressure fluid through the clutch modulator valves and clutch supply lines 164, 166, 1660 from an auxiliary pump 30a. Alternatively, as shown in FIG. 2, clutch pressure fluid could be supplied from the motor circuit by the same primary pump30 that supplies pressure fluid to the motor circuit.

Clutch modulator valves 84, 84a are three-position spools spring-centered to a neutral position 168, but movable under a predetermined pressure drop across the wheel motors to one of two clutch-operating positions 169, 170, depending on whether the wheel motors are operating in their forward or reverse mode. When the clutch modulator spools are in their neutral positions, pressure fluid from auxiliary pump 30a returns to sump through clutch fluid return lines 172, 173.

OPERATION The wheel motor circuit described provides an adaptive control system. Such system senses the tractive condition of primary drive wheels 12 by continuously comparing the speed of engine 14 to the ground speed of the vehicle. Then, unlike prior systems, such system varies the output of the wheel motors 28, 28a and thus the tractive effort supplied by auxiliary drive wheels 23, 23a in response to the changing tractive capability of the primary drive wheels.

Since the assist drive transmission to wheels 23 is at a higher drive ratio than the primary drive transmission to main drive wheels 12, the assist wheels provide no tractive effort under normal drive conditions in which there is no appreciable slippage in the main drive path, even when the assist control system is engaged. However, at any given engine speed, increased slippage in the main drive path results in a slowing of vehicle speed and thus a slowing of the assist wheels. The slowing of assist wheels 23 in comparison to engine speed is thus indicative of slippage in the main drive path and is sensed by the wheel motors through a corresponding increase in differential pressure across such motors, reflecting wheel motor demand. The control circuit responds by directing the available oil supply from pump 30 through the fully open upstream one of control spools 114, 126 to the motor inlets to meet demand. At the same time the downstream one of spools 114, 126 meters return flow back to sump and determines the discharge pressure at the motors. Thus the motor discharge pressure is the control base to which pressures throughout the control circuit are referenced. Therefore the inlet pressure available to drive the motors at any moment is determined by the sum of the differential or demand pressure across the motors and motor discharge pressure.

The assist control system as thus briefly described operates in a manner enabling the wheel motors to transmit torque to the assist wheels only to the extent needed by such wheels toassist the primary drivers when there is slippage in the primary drive path.

Neutral Mode of Operation in the neutral mode, master control valve 100 is spring-centered to its neutral position as shown in FIG. 3. This is the position of the master valve when the operator anticipates that no auxiliary tractive effort will be needed to keep the vehicle moving at the desired speed. In its neutral position, master control valve 100 directs pressure fluid from line 104 to return line 106 back to sump 105. At the same time, pump 30 may be supplying pressure fluid to other hydraulic functions of the vehicle unrelated to the auxiliary wheel motors. In the neutral mode, there is no fluid flow in the motor circuit because wheel motors 28, 280 are mechanically disconnected from wheels 23, 23a by clutches 70, 70a. The lack of differential pressure across the wheel motors maintains clutch modulator valves 84, 84a in their neutral positions to prevent clutch engagement.

Powered Forward and Reverse Modes Operation of the hydraulic motor control circuit in the powered forward mode is illustrated in FIG. 4. Operation of the control circuit in the forward mode is described with the understanding that a similar description and sequence of events applies to operation of the circuit in its reverse mode.

With the vehicle traveling in a forward direction and the operator detecting a need for auxiliary traction assist, he manually shifts master valve 100 to its forward position 102. The master valve admits pressure fluid to forward supply line 108, from which it flows through forward supply port 111, forward supply check valve 117 of bypass passage 116, passages 132 and 113, forward motor ports140, 140a and forward motor supply lines 144, 1440 to the forward inlet ports of wheel motors 28, 28a. At the same time fluid flows through passage 109, passage 1204 of shorting valve 120, passage 125 and lines 146, 1460 to the reverse inlet ports of the motors. All motor ports thus experience a rise in pressure.

As the pressure in the motor circuit rises, such pressure increase is sensed by forward control spool 114 through pilot line 115. At a predetermined pressure the forward spool shifts to block flow through shorting valve 120 and permit flow through valve passage 1140 to the forward motor ports, in response to motor demand. Thus flow through valve passage 114a is combined with the parallel flow through line 1 16 to supply the total flow required at the motor inlets to meet motor demand.

At the same time, pressure buildup in the motor circuit shifts reverse power control spool 126 downwardly, whereby flow from the outlet ports of the fluid motors is blocked at reverse supply check valve 128, but metered back to sump through passage 126a of reverse spool 126.

Under the foregoing conditions, the fluid pressure at the forward motor ports reflects the drive load to the motors. The maximum allowable drive pressure is controlled by the setting of the master control relief valve 107. The fluid pressures at reverse motor ports 142, 142a reflect the reverse power control spool metering schedule as determined by the valve design and resistance of reverse spool spring 122. This meter-out pressure schedule of the reverse control spool 126 depends on the flow rate through the circuit.

In order for the pressure drop across the wheel motors to reflect the demand torque of the rear wheels, clutch modulator spools 84 and 84a must shift at a relatively low pressure drop across the motors to connect the motors to such wheels. Thus, after master control valve 100 is shifted to its forward position and a predetermined low pressure drop across the motors develops, clutch modulator valves 84, 84a shift to one of their two operative positions 169 or 170. Clutch pressure fluid from line 164 flows through valve passages 169a and lines 166, 1660 to clutch pistons 80, a. Pressure buildup at the clutch pistons, slides clutch shafts 78, 78a through their motor cartridges, forcing clutch packs 70, 70a closed to engage the clutches and thus transmit torque through clutch shafts 78, 78a, clutch packs 76, 76a and planetary units 72, 72a to auxiliary drive wheels 23, 23a. Because modulator valves 84, 84:: are sensitive to pressure drop across the wheel motors, a decrease in such pressure drop, such as when the wheel motors approach their flow limits, tends to return the valves to their neutral positions. This causes a reduction in pressure at the clutch pistons and either partial or full clutch disengagement as dictated by clutch fluid pressure and the position of the modulator spools.

As previously indicated, the same sequence of events occurs when master valve is moved to its reverse position 103 to operate the motors in their reverse modes except that charging first occurs through the reverse bypass line and power control spool 126 rather than through the forward bypass line and control spool. Clutch modulator spools 84 and W are adapted to operate in the same manner in both the forward and reverse modes, in both cases operating in response to a predetermined differential pressure across the wheel motors.

The clutch arrangement shown, including rotary clutch piston 80 and shaft 78 with multiple disc oil clutch pack 76, with the clutch shaft being rotatable with the hydraulic motor but slidable through the motor cartridge, permits the clutch to be fully engaged, partially engaged, or disengaged while the vehicle is moving throughout the entire speed range of the vehicle, engine and hydraulic motors.

Overrun Mode of Operation Still referring to FIG. 4, the overrun mode is an extension of the powered mode in the instance when the speed of the wheel motors exceeds available fluid supply flow. This occurs, for example, when the main drive wheels regain traction so that the vehicle gains speed to an extent such that the rear wheels 23 start to backdrive the wheel motors 28, 28a as pumps. Under this condition, assuming the vehicle is still traveling in a forward direction and with the master control valve still in its forward position, the reverse motor port pressures exceed the forward motor port pressures, whereby the motors function as pumps. The resultant pressure drop at the input side of the motors causes forward power control spool 114 to shift upwardly, cutting off flow through passage 114a but permitting flow through check valve 117 and bypass passage 116. The upward shift of spool 114 also permits recirculation of fluid from the motor reverse ports through lines 146, 146a, reverse motor ports 142, 142a, passage 125, shorting valve passage 120 a, passage 109, passage 132, forward motor ports 140, 140a, and forward supply lines 144, 144a back to the forward motor inlet port. At the same time pilot circuit 134 is energizedto shift reverse control spool 126 upwardly, thereby increasing the pressure in the motor circuit to an extent sufficient to ensure adequate charging flow to the motors.

In the overrun condition valve 126 continues to meter flow back to sump. However, the pressure in pilot circuit 134 acting at the spring end of reverse control spool 126 ensures that such spool does not meter flow back to sump without providing the positive recirculation pressure necessary at high flow rates to protect the hydraulic motors against damaging cavitation.

The anticavitation protection circuit just described is a safety feature in the illustrated embodiment which protects the hydraulic wheel motors in the powered mode if the clutches should fail to disengage. However, normally when rear wheels 23 reach a speed such that they begin to back-drive the wheel motors, the pressure differential across the wheel motors drops to a low level, thereby returning the modulator spools 84, 84a to their neutral positions and at least partially disengaging the clutches. If desired, however, modulator valves 84, 84a could be eliminated from the disclosed circuit so that the assist motors would remain connected to the assist wheels at all times when the assist motor circuit is energized. In such case, the anticavitation protection circuit, including pilot circuit 134, would become the primary means of protecting the assist motors against cavitation in the overrun mode.

Preferably the wheel motor drive system is designed so that, in at least low gear and in the absence of slippage in the primary drive path, there is never sufiicient pressure fluid flow available to traction assist motors 28, 28a to drive rear wheels 23, 23a at the same speed that engine 14 drives main drive wheels 12. This feature ensures that the traction assist unit functions only when needed, that is, only as an assist or boost drive when the main drive wheels slip to slow the vehicle and rear wheels to a speed that the traction assist drive unit is capable of matching or exceeding. This means that so long as there is no slippage in the primary drivers 12 or the primary drive train, the assist motor clutches remain disengaged or at most only partially engaged, even though the master control valve remains in a position to energize the wheel motor circuits. However, normally the operator will return the master control valve to its neutral position to deenergize the wheel motor circuit when he anticipates no further need for traction assist.

Retard Mode of Operation As described in conjunction with the overrun mode, the motor control system has an anticavitation motor protection circuit. As shown in FIG. 5, this motor protection circuit enables use of the traction assist wheel motors as pumps purposely to provide a braking effect to the vehicle and thus a retard mode of operation.

Assuming the vehicle is traveling in a forward direction and it is desired to retard or brake forward motion, the operator moves master control valve to its reverse position 103. The initial sequence of events is similar to that which occurs when the reverse powered mode is selected. Initial flow is through reverse supply port 123, reverse supply check valve 128, and reverse spool bypass line 135, passages 137,125, 146 and 146a to the reverse motor inlet ports.

At the same time, flow through passage 139, reverse shorting valve passage 130a and shorting orifice 131 energizes pilot circuit 134 to elevate the fluid pressure in the motor circuit and ensure adequate recirculation of fluid in such circuit. All excess flow returns to sump through relief valve 107. Relief valve 107 may include a manually variable relief pressure setting device 107a as shown, thereby functioning as a manually variable retard control. This feature provides for remote operator control of the desired retard pressure level, or degree of retard, and eliminates the need to tailor the internal valves within assembly 110 to meet various applications. Alternatively, a remote manually variable retard control means could be provided by a suitable variable orifice through master control valve 100.

Variable pressure relief control 107a is also operable in the forward and reverse powered mode to manually vary the motor inlet pressure and therefore the tractive effort at the assist wheels.

Since reverse spool 126 is initially in its shorting range, there is also flow through passage 139, shorting spool passage 130a, passage 113 and lines 144 and 144a toward the forward motor ports. Thus all motor ports in the charge valve assembly are pressurized through reverse supply check valve 128, and motorinduced fluid circulationis shorted from the reverse motor ports back to the forward motor ports through shorting valve portion 130 of reverse control spool 126.

As the motor port pressures exceed a predetermined level of, say, 450 p.s.i., the reverse power control spool shifts to block its shorting passage 130a. As the spool continues to shift, it connects passage 126a to fluid supply line 124 from the master control valve, thereby connecting reverse supply port 123 to reverse motor ports 142, 142a. Simultaneously, forward power control spool 114 shifts toward blockage of its shorting passage a, but stops at a stroke required to maintain a predetermined pressure of, for example, 500 p.s.i. at the forward motor ports 140. The metering of shorting flow through shorting passage 120a of valve 114 from the reverse motor ports back to the forward motor ports continues until the vehicle stops or the master control valve returns to neutral.

Thus, except for very low flow rates, the motor circuit flow rates are independent of supply flow, and are directly dependent upon motor speed. The rate of flow through reverse supply port 123 and reverse spool 126 may be very low, since only enough fluid is required to make up for leakage losses. Most of this make-up flow is returned to the reservoir through the motor case drain tank port 156. Only a small flow through the forward supply balance orifice 118 returns to sump through the master valve.

Throughout the charging of the motor circuit as described, the pilot circuit 134 remains active to maintain the forward motor port pressures high enough to ensure filling of the motors in their pumping modes and while recirculating fluid in the motor circuit at a high flow rate. The maximumpilot pressure is controlled by the setting of pilot relief 136. The pilot circuit is designed to be energized only when needed to ensure adequate pressure for recirculation of flow in the motor circuit.

With the circuit functioning as shown in FIG. 5, rear wheels 23, 23a traveling in a forward direction tend to pump fluid from the reverse motor ports back to the forward motor ports. However, fluid supply pump 30 tries to pump fluid toward the reverse motor ports. Thus the wheel motors pump against a high pressure which provides a strong braking or retarding action to a degree determined bythe pressure setting of valve 107 to slow rotation of the rear wheels and thus the vehicle. This retarding effect can be used to supplement downhill braking or as an emergency braking should the vehicle's main brakes fail.

Of course, the foregoing assumes that wheel motor clutches 70, 70a are engaged in the retard mode. The clutches remain engaged by the high differential pressure across the wheel motors which is transmitted through pilot lines 160, 1604 and 162, 1620 to clutch modulator valves 84, 84a. This high differential pressure shifts such valves toward the right in FIG. to engage the wheel motor clutches. The clutches remain engaged until the braking action of the fluid reduces the speed of the vehicle and thus the pressure drop across the motors to a low level.

The hydraulic traction assist motor control system shown is an open-center system. However, the principles disclosed are also applicable to a closed-center hydraulic power control system.

Having illustrated and described a preferred embodiment of our invention, it should be apparent to those skilled in the art that the same permits of modification in arrangement and detail. We claim as our invention all such modifications as come within the true spirit and scope of the following claims.

We claim:

1. In a drive system for a vehicle including a primary driver, a prime mover, an assist driver, a primary drive train for transmitting power from said prime mover to said primary driver, and an assist drive train for transmitting power from said prime mover to said assist driver,

said assist drive train including a prime mover-driven hydraulic pump means, a hydraulic assist motor means driven by said hydraulic pump means, bydraulic fluid passage means interconnecting said assist motor means and said pump means, and drive means interconnecting said assist motor means and said assist driver,

manually operable control means in said hydraulic passage means for selectively energizing said assist drive train by connecting flow from said pump means to said assist-motor means,

said assist drive train providing when engaged a greater speed reduction to said assist driver than said primary drive train provides to said primary driver in at least the normal low operating speed range of said primary drive train so that said assist driver is normally undriven by said assist motor means and said primary driver provides all of the tractive effort required to move said vehicle at a desired speed in the absence of power slippage in said primary drive train and so that said assist driver supplies tractive effort to supplement the tractive effort of said primary driver when there is power slippage in said primary drive train,

said hydraulic passage means including an assist motor control circuit for ensuring an adequate supply of pressure fluid to the inlet side of said assist motor means to prevent cavitation when said assist drive train is energized.

2. A drive system according to claim 1 wherein said control circuit includes means operable when said assist drive train is engaged to limit the pressure demand of said assist motor means sothat said demand does not exceed the available pressure fluid supply.

3. A drive system according to claim 2 wherein said drive means includes a clutch and said hydraulic control circuit includes clutch-operating means sensitive to differential pressure across said assist motor means, said clutch-operating means being operable to engage said clutch at differential pressures above a predetermined minimum said pressure and being operable to at least partially disengage said clutch at said minimum differential pressure to prevent said assist driver from back-driving said assist motor means.

4. A drive system according to claim 1 wherein said drive means interconnecting said assist motor means and said assist driver includes clutch means sensitive to differential pressure across said assist motor means, said clutch means being operable at a predetermined low said differential pressure to at least partially disengage said assist driver from said assist motor means and thereby prevent cavitation in said assist motor means.

5. A drive system according to claim 1 wherein said hydraulic control circuit has a shorting circuit portion operable under conditions wherein said assist driver tends to back-drive said assist motor means to recirculate pressure fluid from the outlet side to the inlet side of said motor means to provide for adequate filling of said motor means under said conditions.

6. A drive system according to claim 5 including means operable during energization of said shorting circuit to elevate the fluid pressure within said control circuit and thereby ensure an adequate fluid pressure at said motor inlet to prevent cavitation.

7. A drive system according to claim 6 including means selectively operable to vary the elevated pressures within said control circuit when said shorting circuit is energized.

8. A drive system according to claim 5 wherein said shorting circuit includes shorting valve means operable in response to a reduced pressure in said control circuit to energize said shorting circuit.

9. A drive system according to claim 6 wherein said pressure-elevating means includes a pilot circuit operable to elevate the pressure in said control circuit.

10. A drive system according to claim 8 wherein said shorting valve means operates to energize a pilot circuit to increase the fluid pressure in said control circuit when said shorting valve means energizes said shorting circuit.

11. An auxiliary hydraulic drive systemfor a vehicle having a primary drive system includingengine-driven ground-engaging primary drive wheel means and other ground-engaging assist wheel means,

said auxiliary drive system comprising:

hydraulic pump means driven by said engine,

hydraulic wheel motor means for driving said assist wheel means,

said wheel motor means being supplied with'pressure fluid by said pump means,

auxiliary power transmission means interconnecting said pump means, wheel motor means and assist wheel means and under normal operating conditions adapted to drive said assist wheel means at a lower speed than said primary drive system is adapted to drive said primary wheel means in the absence of power slippage in said primary system so that said auxiliary system is operable to sense power slippage in said primary drive system and effective to drive said assist wheel means only when there is slippage in said primary drive system,

and hydraulic control circuit means in said auxiliary drive system responsive to said slippage sensings and operable to protect said wheel motor means against cavitation in the absence of power slippage in said primary drive system sufficient to produce tractive effort at said assist wheel means.

12. A system according to claim 11 wherein said control circuit means includes means for controlling the demand pressure of said wheel motor means and clutch means between said wheel motor means and said assist wheel means, said control circuit being operable when energized to control the extent of engagement of said clutch means in a manner such that said demand pressure of said motor means does not exceed the capacity of said pump means to meet said demand pressure.

13. A system according to claim 11 wherein said hydraulic control circuit includes power control valve means in said control circuit, said control circuit including bypass passage means bypassing said power control valve means from said pump means to said wheel motor means, pilot hydraulic circuit means for applying a control pressure to said power control valve means, and shorting valve means operable to connect said bypass passage means and said pilot circuit means under predetermined pressure conditions in said control circuit in a manner so as to maintain minimum predetermined inlet pressures at said wheel motor means under various operating modes of said wheel motor means.

14. A system according to claim 11 wherein said control circuit includes power control valve means, manually operable master control valve means for selectively energizing said control circuit, said master control valve means and control circuit including means for selectively directing a flow of pressure fluid either to a forward or a reverse motor port of said wheel motor means independently of the directional mode of said primary drive system.

IS. A system according to claim 11 including:

fluid pressure-operated clutch means for transmitting torque from said wheel motor means to said assist wheel means,

clutch modulator means in said control circuit for sensing differential pressure across said wheel motor means,

said modulator meansbeing operable'at differential pressures above a low predetermined said differential pressure to admit'fluid'tosaid'clutchmeans at a pressure sufficient to effect full clutch engagement and being operable below said'low predetermined pressure to effect at least'partial clutch disengagement sufficient to prevent cavitation at said wheel motor means.

16. A system according to claim 15 wherein said clutch means includes friction clutch'm'e'ans and said modulator means is operable at or'below said low predetermined differential pressure'to admit fluid to said clutch means at'apressure sufficientto effect no more than partial clutch engagement so that said wheel motor means is driven at a speed dictated'by the available supply of pressure fluid and independent of the speed of said assist wheel means.

17. An auxiliary drive system according to claim 11 wherein said hydraulic control circuit includes pressure-sensitive control valve means sensitive to discharge pressure at said wheel motor means and operable with said circuit energized 'to meter flow back to said pump means so as to maintain a predetermined discharge pressure at said motor means in 'a manner such that inlet pressure at said motor means at any time back to said pump and recirculating flow from the discharge side to the inlet side of said motor means within said control circuit when the discharge pressure of said motor means exceeds the inlet pressure thereof.

19. An auxiliary drive system according to claim 11 including:

manual selector means for controlling the flow of pressure fluid between said pump and motor means and for alternatively l) interrupting said flow, (2) directing said flow to a side of said motor means to tend to cause forward vehicle movement, and (3) reversing said flow to an opposite side of said motor means to tend to effect reverse vehicle movement,

said selector means being operable with said control circuit to direct fluid flow to said wheel motor means in a retarding directional mode opposing the directional mode of said primary drive system so that said auxiliary drive system has a retarding effect on vehicle movement.

20. An auxiliary drive system according to claim 19 including fluid pressure responsive control valve means in said control circuit operable during fluid flow in said retarding mode to recirculate flow from the outlet side to the inlet side of said motor means so as to tend to increase the inlet pressure at said motor means.

21. An auxiliary drive system according to claim 11 including:

manual selector valve means for directing flow to said hydraulic control circuit and said motor means in a forward or reverse directional mode independently of the directional mode of said primary drive system,

control valve means in said control circuit sensitive to discharge pressure at said motor means and operable at discharge pressures below a predetermined minimum control circuit charge pressure to recirculate fluid flow from the outlet side of said motor means back to the inlet side thereof while restricting return flow from said circuit to said pump means to ensure adequate cavitationpreventing fluid flow through said motor means,

said control valve means being operable at motor discharge pressures above said predetermined minimum pressure to meter fluid flow from the outlet side of said motor means back to said pump means, thereby tending to maintain said predetermined control circuit charge'pressure,

said control circuit including charge pressureregulating means'operable to elevate the level of said predetermined charge pressure when said selector valve means directs fluid flow to said circuit and motor means in a directional mode opposite the directional mode of said primary drive system so that during flow in said opposite directional mode said flow will have a retarding effect on vehicle movement while protecting said motor means against cavitation.-

22. An auxiliary drive system according to claim 21 wherein said charge pressure-regulating means is operable to elevate said charge pressure level whenever said assist wheel means back-drives said wheel motor means.

23. A drive system according to claim 21 including manually operable variable control means selectively operable to elevate the discharge pressure of said motor means upon operation of said auxiliary drive system in a retarding mode.

24. An auxiliary drive system-for a vehicle having a primary drive system including an engine and primary power transmission means for driving primary drive wheel means,

said auxiliary drive system comprising:

traction assist wheel means,

hydraulic wheel motor means and auxiliary power transmission means carried by said assist wheel means for driving the same, pump means driven by said engine and supplying pressure fluid for driving said assist wheel means,

wheel motor fluid control circuit means connected to forward and reverse sides of said motor means,

supply and return passage means connecting forward and reverse sides of said motor control circuit means to said pump means for supplying said motor circuit means with pressure fluid and returning said fluid to said pump means,

manually operated master control valve means l) movable to a first position for directing flow from said pump means through said supply and return passage means in one direction to said forward side of said motor circuit means, (2) movable to a second position for directing flow in an opposite direction to said reverse side of said motor circuit means and (3) movable to a third position to block flow through said supply passage means to said motor circuit means while permitting return flow through said return passage means to said pump means, said motor control circuit means comprising: forward power control valve means sensitive to fluid pressure in said motor circuit means and operable under fluid pressures below a predetermined motor circuit charge pressure to restrict flow to the forward side of said motor means and operable under motor circuit pressures above said predetermined pressure to permit full flow through the forward side of said motor circuit means to the forward side of said motor means when said master control valve is in said first position,

forward bypass passage means for bypassing flow into said motor circuit means past said forward power control valve means to energize said motor circuit means when said master valve means is in said first position and said motor circuit pressure is below said predetermined charge pressure,

reverse power control valve means sensitive to fluid pressure in said motor circuit means and operable under motor discharge pressures below said predetermined charge pressure to block return flow through said reverse valve means from the reverse side of said motor control circuit means to said pump means and-operable in response to discharge pressures above said predetermined pressure to meter flow from the reverse side of said motor circuit means to said pump means when said master control valve means is in said first position, reverse bypass passage means for bypassing flow into the reverse side of said motor circuit means past said reverse power control valve means when said master valve means is in said second position,

and shorting valve means operable when either one of said forward and reverse power control valve means is blocking flow out of said motor circuit means to permit recirculating flow within said motor control circuit so as to provide a positive pressure at the inlet side of said motor means.

25. A system according to claim 24 including pilot fluid circuit means, said pilot circuit means being energized through said shorting valve means when at least one of said forward and reverse power control valve means is in a flow-blocking position and being operable when energized to apply a pilot pressure to said forward and reverse power control valve means tending to resist movement of said power control valve means toward their flow-metering positions and thereby elevate the charge pressure in said control circuit.

26. In a drive system for a vehicle including a primary driver, a normally undriven assist driver, an engine, a primary drive train for transmitting power from said engine to said primary driver, and an assist drive train for transmitting power from said engine to said assist driver,

said assist drive train including an engine-driven hydraulic pump means, a hydraulic assist motor means adapted to be driven by said pump means, a hydraulic fluid circuit for circulating flow between said pump means and said motor means, and drive means interconnecting said assist motor means and said assist driver, manually operable directional control valve means in said circuit for selectively connecting said pump means to said assist motor means in either a forward or a reverse mode of operation of said motor means independent of the directional mode of said primary drive train, and selectively de-energizing said assist drive train, said hydraulic circuit including an assist motor control circuit toward said assist motor means from said directional control valve means with said control circuit including said assist motor means and a power control valve means responsive to fluid pressure in said control circuit at the inlet side of said through a fluid-operated clutch means, said clutch means including a multiple disc friction clutch pack biased to a disengaged condition, a clutch shaft slidable axially through said wheel motor means but rotatable by said motor means to transmit output torque from said motor means, said clutch shaft being operably connected to said clutch pack, a clutch piston at the opposite end of said shaft operable under a predetermined clutch fluid pressure to shift said clutch shaft axially toward said clutch pack to effect clutch engagement. 

1. In a drive system for a vehicle including a primary driver, a prime mover, an assist driver, a primary drive train for transmitting power from said prime mover to said primary driver, and an assist drive train for transmitting power from said prime mover to said assist driver, said assist drive train including a prime mover-driven hydraulic pump means, a hydraulic assist motor means driven by said hydraulic pump means, hydraulic fluid passage means interconnecting said assist motor means and said pump means, and drive means interconnecting said assist motor means and said assist driver, manually operable control means in said hydraulic passage means for selectively energizing said assist drive train by connecting flow from said pump means to said assist motor means, said assist drive train providing when engaged a greater speed reduction to said assist driver than said primary drive train provides to said primary driver in at least the normal low operating speed range of said primary drive train so that said assist driver is normally undriven by said assist motor means and said primary driver provides all of the tractive effort required to move said vehicle at a desired speed in the absence of power slippage in said primary drive train and so that said assist driver supplies tractive effort to supplement the tractive effort of said primary driver when there is power slippage in said primary drive train, said hydraulic passage means including an assist motor control circuit for ensuring an adequate supply of pressure fluid to the inlet side of said assist motor means to prevent cavitation when said assist drive train is energized.
 2. A drive system according to claim 1 wherein said control circuit includes means operable when said assist drive train is engaged to limit the pressure demand of said assist motor means so that said demand does not exceed the available pressure fluid supply.
 3. A drive system according to claim 2 wherein said drive means includes a clutch and said hydraulic control circuit includes clutch-operating means sensitive to differential pressure across said assist motor means, said clutch-operating means being operable to engage said clutch at differential pressures above a predetermined minimum said pressure and being operable to at least partially disengage said clutch at said minimum differential pressure to prevent said assist driver from back-driving said assist motor means.
 4. A drive system according to claim 1 wherein said drive means interconnecting said assist motor means and said assist driver includes clutch means sensitive to differential pressure across said assist motor means, said clutch means being operable at a predetermined low said differential pressure to at least partially disengage said assist driver from said assist motor means and thereby prevent cavitation in said assist motor means.
 5. A drive system according to claim 1 wherein said hydraulic control circuit has a shorting circuit portion operable under conditions wherein said assist driver tends to back-drive said assist motor means to recirculate pressure fluid from the outlet side to the inlet side of said motor means to provide for adequate filling of said motor means under said conditions.
 6. A drive system according to claim 5 including means operable during energization of said shorting circuit to elevate the fluid pressure within said control circuit and thereby ensure an adequate fluid pressure at said motor inlet to prevent cavitation.
 7. A drive system according to claim 6 including means selectively operable to vary the elevated pressures within said control circuit when said shorting circuit is energized.
 8. A drive system according to claim 5 wherein said shorting circuit includes shorting valve means operable in response to a reduced pressure in said control circuit to energize said shorting circuit.
 9. A drive system according to claim 6 wherein said pressure-elevating means includes a pilot circuit operable to elevate the pressure in said control circuit.
 10. A drive system according to claim 8 wherein said shorting valve means operates to energize a pilot circuit to increase the fluid pressure in said control circuit when said shorting valve means energizes said shorting circuit.
 11. An auxiliary hydraulic drive system for a vehicle having a primary drive system including engine-driven ground-engaging primary drive wheel means and other ground-engaging assist wheel means, said auxiliary drive system comprising: hydraulic pump means driven by said engine, hydraulic wheel motor means for driving said assist wheel means, said wheel motor means being supplied with pressure fluid by said pump means, auxiliary power transmission means interconnecting said pump means, wheel motor means and assist wheel means and under normal operating conditions adapted to drive said assist wheel means at a lower speed than said primary drive system is adapted to drive said primary wheel means in the absence of power slippage in said primary system so that said auxiliary system is operable to sense power slippage in said primary drive system and effective to drive said assist wheel means only when there is slippage in said primary drive system, and hydraulic control circuit means in said auxiliary drive system responsive to said slippage sensings and operable to protect said wheel motor means against cavitation in the absence of power slippage in said primary drive system sufficient to produce tractive effort at said assist wheel means.
 12. A system according to claim 11 wherein said control circuit means includes means for controlling the demAnd pressure of said wheel motor means and clutch means between said wheel motor means and said assist wheel means, said control circuit being operable when energized to control the extent of engagement of said clutch means in a manner such that said demand pressure of said motor means does not exceed the capacity of said pump means to meet said demand pressure.
 13. A system according to claim 11 wherein said hydraulic control circuit includes power control valve means in said control circuit, said control circuit including bypass passage means bypassing said power control valve means from said pump means to said wheel motor means, pilot hydraulic circuit means for applying a control pressure to said power control valve means, and shorting valve means operable to connect said bypass passage means and said pilot circuit means under predetermined pressure conditions in said control circuit in a manner so as to maintain minimum predetermined inlet pressures at said wheel motor means under various operating modes of said wheel motor means.
 14. A system according to claim 11 wherein said control circuit includes power control valve means, manually operable master control valve means for selectively energizing said control circuit, said master control valve means and control circuit including means for selectively directing a flow of pressure fluid either to a forward or a reverse motor port of said wheel motor means independently of the directional mode of said primary drive system.
 15. A system according to claim 11 including: fluid pressure-operated clutch means for transmitting torque from said wheel motor means to said assist wheel means, clutch modulator means in said control circuit for sensing differential pressure across said wheel motor means, said modulator means being operable at differential pressures above a low predetermined said differential pressure to admit fluid to said clutch means at a pressure sufficient to effect full clutch engagement and being operable below said low predetermined pressure to effect at least partial clutch disengagement sufficient to prevent cavitation at said wheel motor means.
 16. A system according to claim 15 wherein said clutch means includes friction clutch means and said modulator means is operable at or below said low predetermined differential pressure to admit fluid to said clutch means at a pressure sufficient to effect no more than partial clutch engagement so that said wheel motor means is driven at a speed dictated by the available supply of pressure fluid and independent of the speed of said assist wheel means.
 17. An auxiliary drive system according to claim 11 wherein said hydraulic control circuit includes pressure-sensitive control valve means sensitive to discharge pressure at said wheel motor means and operable with said circuit energized to meter flow back to said pump means so as to maintain a predetermined discharge pressure at said motor means in a manner such that inlet pressure at said motor means at any time is a function of said discharge pressure and the differential pressure across said motor means.
 18. An auxiliary drive system according to claim 17 wherein said control circuit includes means for restricting flow from the discharge side of said motor means back to said pump and recirculating flow from the discharge side to the inlet side of said motor means within said control circuit when the discharge pressure of said motor means exceeds the inlet pressure thereof.
 19. An auxiliary drive system according to claim 11 including: manual selector means for controlling the flow of pressure fluid between said pump and motor means and for alternatively (1) interrupting said flow, (2) directing said flow to a side of said motor means to tend to cause forward vehicle movement, and (3) reversing said flow to an opposite side of said motor means to tend to effect reverse vehicle movement, said selector means being operable with said control circuit to direct fLuid flow to said wheel motor means in a retarding directional mode opposing the directional mode of said primary drive system so that said auxiliary drive system has a retarding effect on vehicle movement.
 20. An auxiliary drive system according to claim 19 including fluid pressure-responsive control valve means in said control circuit operable during fluid flow in said retarding mode to recirculate flow from the outlet side to the inlet side of said motor means so as to tend to increase the inlet pressure at said motor means.
 21. An auxiliary drive system according to claim 11 including: manual selector valve means for directing flow to said hydraulic control circuit and said motor means in a forward or reverse directional mode independently of the directional mode of said primary drive system, control valve means in said control circuit sensitive to discharge pressure at said motor means and operable at discharge pressures below a predetermined minimum control circuit charge pressure to recirculate fluid flow from the outlet side of said motor means back to the inlet side thereof while restricting return flow from said circuit to said pump means to ensure adequate cavitation-preventing fluid flow through said motor means, said control valve means being operable at motor discharge pressures above said predetermined minimum pressure to meter fluid flow from the outlet side of said motor means back to said pump means, thereby tending to maintain said predetermined control circuit charge pressure, said control circuit including charge pressure-regulating means operable to elevate the level of said predetermined charge pressure when said selector valve means directs fluid flow to said circuit and motor means in a directional mode opposite the directional mode of said primary drive system so that during flow in said opposite directional mode said flow will have a retarding effect on vehicle movement while protecting said motor means against cavitation.
 22. An auxiliary drive system according to claim 21 wherein said charge pressure-regulating means is operable to elevate said charge pressure level whenever said assist wheel means back-drives said wheel motor means.
 23. A drive system according to claim 21 including manually operable variable control means selectively operable to elevate the discharge pressure of said motor means upon operation of said auxiliary drive system in a retarding mode.
 24. An auxiliary drive system for a vehicle having a primary drive system including an engine and primary power transmission means for driving primary drive wheel means, said auxiliary drive system comprising: traction assist wheel means, hydraulic wheel motor means and auxiliary power transmission means carried by said assist wheel means for driving the same, pump means driven by said engine and supplying pressure fluid for driving said assist wheel means, wheel motor fluid control circuit means connected to forward and reverse sides of said motor means, supply and return passage means connecting forward and reverse sides of said motor control circuit means to said pump means for supplying said motor circuit means with pressure fluid and returning said fluid to said pump means, manually operated master control valve means (1) movable to a first position for directing flow from said pump means through said supply and return passage means in one direction to said forward side of said motor circuit means, (2) movable to a second position for directing flow in an opposite direction to said reverse side of said motor circuit means and (3) movable to a third position to block flow through said supply passage means to said motor circuit means while permitting return flow through said return passage means to said pump means, said motor control circuit means comprising: forward power control valve means sensitive to fluid pressure in said motor circuit means and operable under fluid pressures beloW a predetermined motor circuit charge pressure to restrict flow to the forward side of said motor means and operable under motor circuit pressures above said predetermined pressure to permit full flow through the forward side of said motor circuit means to the forward side of said motor means when said master control valve is in said first position, forward bypass passage means for bypassing flow into said motor circuit means past said forward power control valve means to energize said motor circuit means when said master valve means is in said first position and said motor circuit pressure is below said predetermined charge pressure, reverse power control valve means sensitive to fluid pressure in said motor circuit means and operable under motor discharge pressures below said predetermined charge pressure to block return flow through said reverse valve means from the reverse side of said motor control circuit means to said pump means and operable in response to discharge pressures above said predetermined pressure to meter flow from the reverse side of said motor circuit means to said pump means when said master control valve means is in said first position, reverse bypass passage means for bypassing flow into the reverse side of said motor circuit means past said reverse power control valve means when said master valve means is in said second position, and shorting valve means operable when either one of said forward and reverse power control valve means is blocking flow out of said motor circuit means to permit recirculating flow within said motor control circuit so as to provide a positive pressure at the inlet side of said motor means.
 25. A system according to claim 24 including pilot fluid circuit means, said pilot circuit means being energized through said shorting valve means when at least one of said forward and reverse power control valve means is in a flow-blocking position and being operable when energized to apply a pilot pressure to said forward and reverse power control valve means tending to resist movement of said power control valve means toward their flow-metering positions and thereby elevate the charge pressure in said control circuit.
 26. In a drive system for a vehicle including a primary driver, a normally undriven assist driver, an engine, a primary drive train for transmitting power from said engine to said primary driver, and an assist drive train for transmitting power from said engine to said assist driver, said assist drive train including an engine-driven hydraulic pump means, a hydraulic assist motor means adapted to be driven by said pump means, a hydraulic fluid circuit for circulating flow between said pump means and said motor means, and drive means interconnecting said assist motor means and said assist driver, manually operable directional control valve means in said circuit for selectively connecting said pump means to said assist motor means in either a forward or a reverse mode of operation of said motor means independent of the directional mode of said primary drive train, and selectively de-energizing said assist drive train, said hydraulic circuit including an assist motor control circuit toward said assist motor means from said directional control valve means with said control circuit including said assist motor means and a power control valve means responsive to fluid pressure in said control circuit at the inlet side of said motor means for controlling the flow of pressure fluid from said pump means into said control circuit and responsive to fluid pressure in said control circuit at the outlet side of said assist motor means for controlling the return flow of pressure fluid from said control circuit to said pump means to ensure adequate fluid pressurization of said control circuit in all operational modes of said assist motor means to prevent cavitating said motor means.
 27. A system according to claim 23 wherein said wheel motor means drives said assist wheel means throuGh a fluid-operated clutch means, said clutch means including a multiple disc friction clutch pack biased to a disengaged condition, a clutch shaft slidable axially through said wheel motor means but rotatable by said motor means to transmit output torque from said motor means, said clutch shaft being operably connected to said clutch pack, a clutch piston at the opposite end of said shaft operable under a predetermined clutch fluid pressure to shift said clutch shaft axially toward said clutch pack to effect clutch engagement. 